This invention relates to apparatus for examining the human body with penetrating radiation such as x-ray and gamma radiation. Although novel features, discussed in detail hereinafter, are applicable to apparatus for examining the entire body or parts thereof such as the torso, for convenience, the novel features will be described primarily in reference to apparatus for examining human breasts.
The basic features of one type of examination apparatus in which the invention may be employed are illustrated in co-pending U.S. Patent application Ser. No. 600,874, filed July 31, 1975, now U.S. Pat. No. 3,973,126, assigned to the assignee of this application. The disclosure of said application is incorporated herein by reference. Background information on the state of the art may be obtained from an article entitled "Image Reconstruction From Projections" by R. Gordon, G. T. Herman and S. A. Johnson in Scientific American, October, 1975, Vol. 233, No. 4, p. 56.
Detection of breast tumors is a process which depends on being able to differentiate radiation absorption by normal tissue and absorption by the tumor or other malignancy which has substantially equal density. Heretofore, radiographic films, fluoroscopic screens and xeroradiographic plates were used as the x-ray detecting devices but none of these devices had sufficient sensitivity to differentiate between normal and abnormal tissue.
Recently developed procedures for soft tissue differentiation with x-rays, using a scanning method, are described in U.S. Pat. Nos. 3,778,614, 3,867,634 and 3,881,110. As described in the patents, one or more finely collimated gamma ray or x-ray beams from a common source are directed through the examination subject and to a detector array on the side opposite of the subject from the source. In one scheme, the source and detector are translated back and forth and at the end of each translation the source and detector are incremented rotationally. Signals corresponding with detected density variations are fed into a computer which, when information from the whole scan is complete, produces data representative of density variations in the transverse plane. The data may be used to control a suitable display device such as a picture tube which enables visualization of the reconstructed image. The procedure is a variation of what is commonly called tomography.
Calculations of the image representative data is simplified if, as in accordance with some of the prior patents, the skin-to-air interface and density variations across the scanning beam are reduced by surrounding the body part in a fluid having a density or x-ray transmissibility substantially equal to that of tissue. Water meets this requirement substantially as mentioned in above cited U.S. Pat No. 3,881,110. On the other hand, in prior art computerized tomography apparatus for examining portions of the body other than breasts and when examining such portions with apparatus using the new features and principles of body scanning with radiation disclosed herein, surrounding the examination subject with water is not always feasible but computerized image reconstruction is still obtainable if proper allowances are made. Heretofore, x-ray scanning apparatus had not achieved the levels of image definition and examination speed for practical mass screening of female breasts nor was there any apparatus available that was dedicated to making breast examinations or examinations by other body portions by scanning techniques that gave the subject x-ray dosages closer to the lower dosages which are theoretically obtainable.